Electric power tools of this kind are known. The electric motor of electric power tools of this kind is typically supplied with a generally single-phase AC mains voltage by phase gating control. This is done in the manner of phase gating control by the electric power switch being “triggered”, that is to say switched to the on state, by application of a control potential or control current (triggering pulse) by the electronic control device, so that a motor current can flow as a result of the voltage applied to the electric motor. One problem which frequently arises is that of so-called “faulty triggering”, that is to say that the power switch has not been switched to the on state or has returned to the off state, for example because the triggering moment did not occur after, but rather shortly before, a zero crossing of the motor current, and therefore the power switch was deactivated, that is to say switched to the off state, again by the following current zero crossing.
In order to ensure reliable operation and rotation of the electric motor, which is a universal motor in particular, it is necessary to ensure that the power switch is duly switched to the on state in accordance with prespecified powers as intended at the correct time during a half-wave of the AC voltage, and also remains in the on state.
To this end, it is feasible, for example, for triggering to be monitored by measuring the voltage across the power switch. It would also be feasible for the motor current to be measured; however, a low-resistance measurement resistor which, for its part, would in turn require an amplifier arrangement for the measurement signal, would have to be used for this purpose. Both these measures are complex and costly to implement in respect of hardware. Outputting several triggering pulses one after the other at a predefined time interval would result in the unnecessary consumption of triggering current in the case in which the power switch is duly in the on state. In addition, a power supply part forming the several triggering pulses would have to be of correspondingly complex design.
The present invention is based on the object of providing an electric power tool of the type described in the introductory part in which reliable actuation of the electric motor as intended and as required is ensured in an economical manner.
According to the invention, this object is achieved by an electric power tool of said type in that the electronic control device comprises a bias voltage output and a detection input which are connected to one another and to that side of the circuit breaker which faces the electric motor via a voltage divider which has a summation point, and the control device is further designed such that the potential across the detection input after respective triggering of the power switch is monitored, and, on the basis of this, a check is made as to whether the power switch is on, and that it is triggered again if the power switch was not on or had returned to the off state during monitoring, and that this check and possibly renewed triggering of the power switch is repeated within a half-wave of the AC voltage.
Monitoring triggering of the power switch in this way using a circuit arrangement having two resistors in conjunction with a bias voltage output and a detection input of the electronic control device, which typically comprises a microcontroller, is associated with an extremely low level of possible costs of implementation in respect of hardware. In addition, the requirements made of programming of the electronic control device which is required for this purpose are relatively low. All that is necessary following a triggering pulse is that preferably continuous, that is to say not only intermittent, monitoring of the potential across the detection input of the control device be carried out. To this end, a signal is available at the voltage divider and therefore at the detection input immediately after triggering of the power switch, said signal, in this way, allowing basically immediate assessment of the state of the power switch (on or off).
Therefore, according to the invention, a check as to whether the power switch has been triggered as intended and also continuously remains in the on state is made by the control device immediately after triggering of the power switch. If this is not the case, this is detected by virtue of a change in signal at the detection input of the control device and the further control measures can be executed, specifically renewed triggering of the power switch as required. The potential across the detection input is again monitored immediately after this and a check is made as to whether this further triggering of the power switch leads to continuous “success” or whether the power switch returns to the off state again, and therefore still further triggering is initiated.
The power switch is advantageously a triac. The invention can also be advantageously used in multi-phase systems.
The check and, if required, triggering of the power switch are carried out at most ten times, in particular at most eight times, in particular at most six times, and further particularly at most five times, within a half-wave according to one embodiment of the invention. It has proven advantageous for the number of checks and, if required, triggering operations of the power switch to be performed at most x times, where x=Thalf-wave/Ttriggering sequence. In this case, Ttriggering sequence denotes the time interval between two triggering operations which is predefined in the control arrangement.
If it is established during the check that the power switch is off, it may prove advantageous for post-triggering to be performed immediately, that is to say as rapidly as possible, as soon as the check of the signal at the detection input has shown that the power switch is off. This may be the case, in particular, when the power switch is initially triggered in the middle of a half-wave.
However, it may also prove advantageous, in a development of the invention, for the electronic control device to be designed such that post-triggering takes place only after a predefined time interval, so that the triggering sequence or Ttriggering sequence lasts for 5 to 500 μs, in particular 100 to 500 μs, in particular 150 to 400 μs and preferably 200 to 300 μs. This may be the case, in particular, when the power switch is initially triggered at the beginning of a half-wave, for example when the current zero crossing has not yet taken place (in this case, immediate post-triggering would not lead to the desired result since the following current zero crossing would re-open the power switch).
The triggering period of the power switch lasts for preferably 5 to 40 μs, in particular 15 to 30 μs.
The electronic control device advantageously comprises a synchronization input in order to detect the zero crossing of the respective half-wave. This is intended to prevent the power switch from being triggered too early, that is to say, for example, during a time interval when the actual motor current is “lagging”, that is to say is still ahead of the respective current zero crossing, on account of inductive loads of the electric motor. In such a case, the power switch could be switched to the on state by triggering, but (as already mentioned above) it would immediately return to the off state again at the subsequent zero crossing of the motor current. Preferably exact triggering of the power switch in relation to the half-wave of the relevant phase also proves advantageous in order to actuate the electric motor as required.
The electronic control device is further advantageously designed such that the bias voltage output is operated with a negative control voltage (low), in particular of −5 volts, during a positive half-wave, and with a comparatively higher potential, in particular zero volt, (high) during a negative half-wave.
The present invention also relates to a method for operating an electric power tool having the features of claim 10.